This invention relates to analog-to-digital (A/D) converters for use in pacemakers and other implantable medical devices, and more particularly to systems and methods for low-power A/D conversion for use in such devices.
Historically, the processing of analog signals within electronic devices such as implantable cardiac pacemakers has been performed by using conventional analog techniques, employing active components (transistors in one form or another) operating in a linear mode along with associated passive components controlling the overall circuit characteristics (gain, bandpass, etc.). Even though there has long been circuitry which could convert analog signals into digital form, A/D converters have until very recently been considered impractical for use in implantable devices.
One of the prime considerations for the pacemaker circuit designer is circuit current consumption. To achieve maximum device life, all circuitry must consume minimal current from the internal power cell. Typically, A/D converter circuits consume excessive amounts of current for use in this application.
In recent years, ultra-low-power digital microprocessors have become available and their use in cardiac pacemakers is becoming commonplace. Because of this, the processing and storage capabilities of pacemakers have been significantly enhanced, such that "software" control of analog input signal processing, including processing of the intracardiac ECG, has become a real possibility, along with development of algorithms for recognizing and detecting specific cardiac arrhythmias. It has been known since the advent of exercise-responsive pacemakers at least a decade ago that practical implementation of algorithms responsive to physiological parameters indicative of exercise would virtually require digital processing, and that such processing would require not only low-power digital logic devices, but also a low-power A/D converter. The inadequacy of available A/D converters was and continues to be compounded by other electrical and physical constraints imposed on pacemakers and other implantable devices, including the need for low-voltage operation, minimum circuit parts count, circuit reproducibility, compatibility with active devices available for such applications, acceptable analog bandwidths, minimum "active trims", and high finished-circuit yields.
Various attempts have been made to solve one or more of these problems, and various types of A/D converters have been proposed for use in pacemakers or other implantable devices, as exemplified by the following U.S. and foreign patent references:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 4,409,984 Dick Oct. 18, 1983 4,467,807 Bornzin Aug. 28, 1984 4,543,953 Slocum et al. Oct. 1, 1985 4,543,954 Cook et al. Oct. 1, 1985 4,757,816 Ryan et al. Jul. 9, 1988 ______________________________________ Foreign Patent Document Applicant Published ______________________________________ GB 2,026,870A Medtronic Feb. 13, 1980 ______________________________________
An early proposal for a low-power A/D converter for a pacemaker is described in the above-referenced UK patent application No. GB 2,026,870A of Medtronic, particularly FIGS. 8 and 10 therein. In the disclosed device, an analog voltage to be converted to digital form is applied to the input of a voltage-controlled oscillator (VCO) whose output is applied to the input of a counter which counts up for a fixed period of time. The VCO input is then switched over to a reference voltage and the counter is placed in downcount mode. A second counter counts clock pulses from a second clock source until the first counter counts down to zero, such that the count in the second counter is directly proportional to the unknown input voltage. The second clock source is said to impose a minimal drain upon the pacer battery. The VCO is turned off during a wait mode for the A/D converter to conserve power. The converter rests in wait mode, with the last converted digital word resting in the second counter, until it receives a strobe pulse. Although the VCO duty cycle is controlled to reduce A/D converter power consumption, the VCO is on during the entire digitizing process, i.e., the up-count and down-count cycles of its associated counter, for each sample. The application indicates that various analog values can be converted to digital form, such as power supply voltage and the P and R waves of the patient's EKG.
The above-referenced patent references also provide evidence of various problems associated with A/D conversion in pacemakers. For example, Dick points out that A/D converters are rather costly, and proposes instead a technique of analog-to-FM-to-digital-to-analog conversion without using an A/D converter, to obtain a digital representation of an analog signal such as an ECG signal or other analog physiological parameter originating in a patient. The technique includes counting high-rate clock pulses during four FM periods to produce a count that is proportional to the time per FM period, and then calculating the reciprocal of time to produce a digital number that is proportional to frequency and, hence, to the instantaneous amplitude of the analog input voltage.
A/D converters were also avoided by Slocum et al. in U.S. Pat. No. 4,543,953, in which the stated general object of the invention was to transmit a reliable high-fidelity analog signal from the implant without excessive power consumption. Slocum et al. recognized the desirability of the intracardiac electrogram (ICEG) signal as well as the extreme difficulty in reliably transmitting the ICEG signal given the battery power constraints of the pacer. Slocum et al. further recognized that A/D conversion of the ICEG signal would be ideal given the proper sampling rate because of the inherent fidelity of the signal, but rejected the technique because A/D converters consume excessive power. The proposed alternative is an analog telemetry system.
A telemetry system is also proposed in the later issued patent to Ryan et al. as a simple technique of transmitting internal signals such as the intracardiac electrogram from an implanted pacer in a digital transmission rather than in an analog transmission, "but which also eliminates the usual steps needed in an analog-to-digital conversion system."